__NOTICE__: This document is a work-in-progress for researchers and implementers. This is an accompanying document to [Ethereum 2.0 Phase 0 -- The Beacon Chain](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md) that describes the expected actions of a "validator" participating in the Ethereum 2.0 protocol.
This document represents the expected behavior of an "honest validator" with respect to Phase 0 of the Ethereum 2.0 protocol. This document does not distinguish between a "node" (ie. the functionality of following and reading the beacon chain) and a "validator client" (ie. the functionality of actively participating in consensus). The separation of concerns between these (potentially) two pieces of software is left as a design decision that is out of scope.
A validator is an entity that participates in the consensus of the Ethereum 2.0 protocol. This is an optional role for users in which they can post ETH as collateral and verify and attest to the validity of blocks to seek financial returns in exchange for building and securing the protocol. This is similar to proof of work networks in which a miner provides collateral in the form of hardware/hash-power to seek returns in exchange for building and securing the protocol.
All terminology, constants, functions, and protocol mechanics defined in the [Phase 0 -- The Beacon Chain](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md) doc are requisite for this document and used throughout. Please see the Phase 0 doc before continuing and use as a reference throughout.
Validator public keys are [G1 points](https://github.com/ethereum/eth2.0-specs/blob/master/specs/bls_signature.md#g1-points) on the [BLS12-381 curve](https://z.cash/blog/new-snark-curve). A private key, `privkey`, must be securely generated along with the resultant `pubkey`. This `privkey` must be "hot", that is, constantly available to sign data throughout the lifetime of the validator.
#### BLS withdrawal key
A secondary withdrawal private key, `withdrawal_privkey`, must also be securely generated along with the resultant `withdrawal_pubkey`. This `withdrawal_privkey` does not have to be available for signing during the normal lifetime of a validator and can live in "cold storage".
In phase 0, all incoming validator deposits originate from the Ethereum 1.0 PoW chain. Deposits are made to the [deposit contract](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#ethereum-10-deposit-contract) located at `DEPOSIT_CONTRACT_ADDRESS`.
* Pack the validator's [initialization parameters](#initialization) into `deposit_input`, a [`DepositInput`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#depositinput) SSZ object.
* Send a transaction on the Ethereum 1.0 chain to `DEPOSIT_CONTRACT_ADDRESS` executing `deposit` along with `serialize(deposit_input)` as the singular `bytes` input along with a deposit `amount` in Gwei.
_Note_: Deposits made for the same `pubkey` are treated as for the same validator. A singular `Validator` will be added to `state.validator_registry` with each additional deposit amount added to the validator's balance. A validator can only be activated when total deposits for the validator pubkey meet or exceed `MAX_DEPOSIT_AMOUNT`.
Deposits cannot be processed into the beacon chain until the eth1.0 block in which they were deposited or any of its descendants is added to the beacon chain `state.eth1_data`. This takes _a minimum_ of `ETH1_FOLLOW_DISTANCE` eth1.0 blocks (~4 hours) plus `ETH1_DATA_VOTING_PERIOD` epochs (~1.7 hours). Once the requisite eth1.0 data is added, the deposit will normally be added to a beacon chain block and processed into the `state.validator_registry` within an epoch or two. The validator is then in a queue to be activated.
Once a validator has been processed and added to the beacon state's `validator_registry`, the validator's `validator_index` is defined by the index into the registry at which the [`ValidatorRecord`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#validator) contains the `pubkey` specified in the validator's deposit. A validator's `validator_index` is guaranteed to not change from the time of initial deposit until the validator exits and fully withdraws. This `validator_index` is used throughout the specification to dictate validator roles and responsibilities at any point and should be stored locally.
In normal operation, the validator is quickly activated at which point the validator is added to the shuffling and begins validation after an additional `ENTRY_EXIT_DELAY` epochs (25.6 minutes).
The function [`is_active_validator`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#is_active_validator) can be used to check if a validator is active during a given epoch. Usage is as follows:
_Note_: There is a maximum validator churn per finalized epoch so the delay until activation is variable depending upon finality, total active validator balance, and the number of validators in the queue to be activated.
A validator has two primary responsibilities to the beacon chain -- [proposing blocks](block-proposal) and [creating attestations](attestations-1). Proposals happen infrequently, whereas attestations should be created once per epoch.
A validator is expected to propose a [`BeaconBlock`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#beaconblock) at the beginning of any slot during which `get_beacon_proposer_index(state, slot)` returns the validator's `validator_index`. To propose, the validator selects the `BeaconBlock`, `parent`, that in their view of the fork choice is the head of the chain during `slot`. The validator is to create, sign, and broadcast a `block` that is a child of `parent` and that executes a valid [beacon chain state transition](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#beacon-chain-state-transition-function).
There is one proposer per slot, so if there are N active validators any individual validator will on average be assigned to propose once per N slots (eg. at 312500 validators = 10 million ETH, that's once per ~3 weeks).
Set `block.slot = slot` where `slot` is the current slot at which the validator has been selected to propose. The `parent` selected must satisfy that `parent.slot < block.slot`.
_Note:_ there might be "skipped" slots between the `parent` and `block`. These skipped slots are processed in the state transition function without per-block processing.
##### Parent root
Set `block.parent_root = hash_tree_root(parent)`.
##### State root
Set `block.state_root = hash_tree_root(state)` of the resulting `state` of the `parent -> block` state transition.
_Note_: To calculate `state_root`, the validator should first run the state transition function on an unsigned `block` containing a stub for the `state_root`. It is useful to be able to run a state transition function that does _not_ validate signatures for this purpose.
`block.eth1_data` is a mechanism used by block proposers vote on a recent Ethereum 1.0 block hash and an associated deposit root found in the Ethereum 1.0 deposit contract. When consensus is formed, `state.latest_eth1_data` is updated, and validator deposits up to this root can be processed. The deposit root can be calculated by calling the `get_deposit_root()` function of the deposit contract using the post-state of the block hash.
*`vote.eth1_data.block_hash` is the hash of an eth1.0 block that is (i) part of the canonical chain, (ii) >= `ETH1_FOLLOW_DISTANCE` blocks behind the head, and (iii) newer than `state.latest_eth1_data.block_data`.
* Let `best_vote` be the member of `D` that has the highest `vote.eth1_data.vote_count`, breaking ties by favoring block hashes with higher associated block height.
* Let `block_hash = best_vote.eth1_data.block_hash`.
* Let `deposit_root = best_vote.eth1_data.deposit_root`.
* Set `block.eth1_data = Eth1Data(deposit_root=deposit_root, block_hash=block_hash)`.
Up to `MAX_PROPOSER_SLASHINGS` [`ProposerSlashing`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#proposerslashing) objects can be included in the `block`. The proposer slashings must satisfy the verification conditions found in [proposer slashings processing](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#proposer-slashings-1). The validator receives a small "whistleblower" reward for each proposer slashing found and included.
Up to `MAX_ATTESTER_SLASHINGS` [`AttesterSlashing`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#attesterslashing) objects can be included in the `block`. The attester slashings must satisfy the verification conditions found in [Attester slashings processing](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#attester-slashings-1). The validator receives a small "whistleblower" reward for each attester slashing found and included.
Up to `MAX_ATTESTATIONS` aggregate attestations can be included in the `block`. The attestations added must satisfy the verification conditions found in [attestation processing](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#attestations-1). To maximize profit, the validator should attempt to create aggregate attestations that include singular attestations from the largest number of validators whose signatures from the same epoch have not previously been added on chain.
Up to `MAX_DEPOSITS` [`Deposit`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#deposit) objects can be included in the `block`. These deposits are constructed from the `Deposit` logs from the [Eth1.0 deposit contract](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#ethereum-10-deposit-contract) and must be processed in sequential order. The deposits included in the `block` must satisfy the verification conditions found in [deposits processing](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#deposits-1).
Up to `MAX_EXITS` [`Exit`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#exit) objects can be included in the `block`. The exits must satisfy the verification conditions found in [exits processing](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#exits-1).
A validator is expected to create, sign, and broadcast an attestation during each epoch. The slot during which the validator performs this role is any slot at which `get_crosslink_committees_at_slot(state, slot)` contains a committee that contains `validator_index`.
A validator should create and broadcast the attestation halfway through the `slot` during which the validator is assigned -- that is `SLOT_DURATION * 0.5` seconds after the start of `slot`.
#### Attestation data
First the validator should construct `attestation_data`, an [`AttestationData`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#attestationdata) object based upon the state at the assigned slot.
##### Slot
Set `attestation_data.slot = slot` where `slot` is the current slot of which the validator is a member of a committee.
Set `attestation_data.shard = shard` where `shard` is the shard associated with the validator's committee defined by `get_crosslink_committees_at_slot`.
Set `attestation_data.beacon_block_root = hash_tree_root(head)` where `head` is the validator's view of the `head` block of the beacon chain during `slot`.
Set `attestation_data.epoch_boundary_root = hash_tree_root(epoch_boundary)` where `epoch_boundary` is the block at the most recent epoch boundary in the chain defined by `head` -- i.e. the `BeaconBlock` where `block.slot == get_epoch_start_slot(head.slot)`.
Set `attestation_data.shard_block_root = ZERO_HASH`.
_Note:_ This is a stub for phase 0.
##### Latest crosslink root
Set `attestation_data.latest_crosslink_root = state.latest_crosslinks[shard].shard_block_root` where `state` is the beacon state at `head` and `shard` is the validator's assigned shard.
Set `attestation_data.justified_block_root = hash_tree_root(justified_block)` where `justified_block` is the block at `state.justified_epoch` in the chain defined by `head`.
Next the validator creates `attestation`, an [`Attestation`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#attestation) object.
Set `attestation.data = attestation_data` where `attestation_data` is the `AttestationData` object defined in the previous section, [attestation data](#attestation-data).
_Note_: Calling `get_attestation_participants(state, attestation.data, attestation.aggregation_bitfield)` should return a list of length equal to 1, containing `validator_index`.
The beacon chain shufflings are designed to provide a minimum of 1 epoch lookahead on the validator's upcoming responsibilities of proposing and attesting dictated by the shuffling and slot.
`get_crosslink_committees_at_slot` is designed to be able to query slots in the next epoch. When querying slots in the next epoch there are two options -- with and without a `registry_change` -- which is the optional third parameter of the function. The following helper can be used to get the potential crosslink committees in the next epoch for a given `validator_index`. This function returns a list of 2 shard committee tuples.
`get_next_epoch_committee_assignments` should be called at the beginning of each epoch to plan for the next epoch. A validator should always plan for both values of `registry_change` as a possibility unless the validator can concretely eliminate one of the options. Planning for a future shuffling involves noting at which slot one might have to attest and propose and also which shard one should begin syncing (in phase 1+).
"Slashing" is the burning of some amount of validator funds and immediate ejection from the active validator set. In Phase 0, there are two ways in which funds can be slashed -- [proposer slashing](#proposer-slashing) and [attester slashing](#attester-slashing). Although being slashed has serious repercussions, it is simple enough to avoid being slashed all together by remaining _consistent_ with respect to the messages a validator has previously signed.
_Note_: Signed data must be within a sequential `Fork` context to conflict. Messages cannot be slashed across diverging forks. If the previous fork version is 1 and the chain splits into fork 2 and 102, messages from 1 can slashable against messages in forks 1, 2, and 102. Messages in 2 cannot be slashable against messages in 102 and vice versa.
To avoid "proposer slashings", a validator must not sign two conflicting [`ProposalSignedData`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#proposalsigneddata) where conflicting is defined as having the same `slot` and `shard` but a different `block_root`. In phase 0, proposals are only made for the beacon chain (`shard == BEACON_CHAIN_SHARD_NUMBER`).
_In phase 0, as long as the validator does not sign two different beacon chain proposals for the same slot, the validator is safe against proposer slashings._
If the software crashes at some point within this routine, then when the validator comes back online the hard disk has the record of the _potentially_ signed/broadcast block and can effectively avoid slashing.
To avoid "attester slashings", a validator must not sign two conflicting [`AttestationData`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#attestationdata) objects where conflicting is defined as a set of two attestations that satisfy either [`is_double_vote`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#is_double_vote) or [`is_surround_vote`](https://github.com/ethereum/eth2.0-specs/blob/master/specs/core/0_beacon-chain.md#is_surround_vote).
1. Save a record to hard disk that an attestation has been signed for source -- `attestation_data.justified_epoch` -- and target -- `slot_to_epoch(attestation_data.slot)`.
If the software crashes at some point within this routine, then when the validator comes back online the hard disk has the record of the _potentially_ signed/broadcast attestation and can effectively avoid slashing.
